But how on earth did plankton get from the surface of the sea all the way to the ISS?

The most obvious guess is that they were stowaways, having somehow got on the surface of a Russian rocket before it took off for the space station. But Vladimir Solovyev, head of the Russian portion of the ISS, says that’s not possible. “[Plankton in] such phases of development is found on the surface of the ocean. It isn’t characteristic to Baikonur,” Solovyev told RT News.

Solovyev is referring to Baikonur Cosmodrome in Kazakhstan, from where the Russian pieces of ISS were launched in 1998—and from where crew and cargo deliveries to the ISS still take off. Baikonur is hundreds of kilometers from the nearest body of saltwater, the Caspian Sea.

The alternative theory? “It turns out that there are some rising air currents, which settle on the surface of the station,” Solovyev says. In other words, a really strong wind lofted plankton up through the Earth’s atmosphere, beyond gravity’s pull, and into space.

The ISS is in the thermosphere. (NASA)

That might not be as far-fetched as it sounds. The other microorganisms scientists have found thriving on spaceship surfaces had to get there somehow. Wind is the main way microbes find their way into the upper atmosphere. Heat from the sun causes big air masses to rise, carrying microorganisms and other particles as they do. In theory, intense pressure and high enough temperature could counteract gravity, allowing relatively heavy things like plankton particles into the thermosphere, the part of the atmosphere where ISS hovers.

If it turns out that it was indeed plankton that was sliming the ISS windows, it could offer new insight into how microorganisms behave in space. Understanding that is a step toward developing colonies of beneficial microorganisms that could allow people to live in space for longer periods of time. While this sort of research has so far mainly been focused on plants, scientists say that bacteria and plankton could also help with things like vitamin production, water recycling, air decontamination, and waste management.